Nuclear weapons either produce energy through nuclear fission (fission bombs) or a combination of fission and fusion (thermonuclear or hydrogen bombs). In both cases, nuclear reactions with neutrons cause the uranium or plutonium fuel to fission into two smaller nuclei, called fission fragments. These fragments are radioactive, and can be detected by their characteristic decay radiation.

If we detect these fission fragments, we know that a nuclear explosion occurred. And that’s where “sniffer” planes come in.

Enter ‘sniffer’ planes

Since 1947, the United States Air Force has operated a nuclear explosions detection unit.

The current fleet uses the WC-135 Constant Phoenix. The aircraft fly through clouds of radioactive debris to collect air samples and catch dust. By measuring their decay, fission fragments can be detected in minute quantities.

The crew are kept safe using filters to scrub cabin air. Radiation levels are monitored using personal measuring devices for each crew member.

On the ground, the Comprehensive Test Ban Treaty Organisation (CTBTO) operates 80 ground-based monitoring stations across the globe that constantly monitor the air for fission products that have dispersed through the atmosphere.

Japan and South Korea operate their own radiation monitoring networks. These networks will also presumably be looking for signatures of the latest North Korean test.

CTBTO radiation monitoring system.

What can fission fragments tell us?

When a nuclear test occurs underground, the fission fragments are trapped except for noble gasses.

Because noble gasses don’t react chemically (except in extreme cases), they diffuse through the rock and eventually escape, ready to be detected.

In particular, some radioactive isotopes of the chemical element xenon are useful due to the fact these isotopes of xenon don’t appear in the atmosphere naturally, have decay times that are neither too long nor too short, and are produced in large quantities in a nuclear explosion. If you see these isotopes, you know a nuclear test occurred.

North Korean leader Kim Jong Un and weapon as reported by KCNA. Is this what exploded? Sniffer planes can’t tell us this.KCNA

Something happened during this test that has people excited — there was an additional magnitude 4.1 tremor around eight minutes after the initial tremor, according to the United States Geological Survey. Among other things, this may indicate that the tunnel containing the bomb collapsed. If this happened, then other fission products and other radioactive isotopes could escape as dust particles.

This might have been accidental or deliberate (to provide proof to international viewers), but in either case, we may learna lot, depending on how fast the sniffer planes arrived and how much dust was released.

For example, by looking at the probability of seeing fission fragments with different masses, the composition of the fission fuel could be determined. We could also learn about the composition of the rest of the bomb. These facts are things that nuclear states keep very secret.

What can’t they tell us?

The amount of information a sniffer plane can determine depends on how much material was released from the test site, how quickly it was released (due to nuclear decay) and how rapidly the sniffer plane got into place.

But fission fragment measurements probably can’t tell us whether the bomb tested was small enough to fit on an Intercontinental Ballistic Missile (ICBM). After all, it’s easy enough for North Korea to show a casing in a staged photograph and blow up something else.